This invention relates to a process for producing polymer ion-exchange membranes as solid polymer electrolyte membranes suitable for use in fuel cells.
The invention also relates to a process for producing polymer ion-exchange membranes having outstanding electrical conductivity and oxidation resistance that are solid polymer electrolyte membranes suitable for use in fuel cells.
Fuel cells using solid polymer electrolyte ion-exchange membranes feature high energy density, so they hold promise for use as power supplies to automobiles or as convenient auxiliary power supplies. One of the most critical aspects of the fuel cell technology is the development of polymer ion-exchange membranes having outstanding characteristics.
In a solid polymer ion-exchange membrane fuel cell, the ion-exchange membrane serves to conduct protons and it also plays the part of diaphragm which prevents direct mixing of the fuel hydrogen or methanol with the oxidant air (oxygen). The ion-exchange membrane has several requirements of an electrolyte to meet: large ion-exchange capacity; sufficient chemical stability of the membrane to allow for prolonged application of an electric current, in particular, high resistance (oxidation resistance) to hydroxide radicals and the like which are principal factors that contribute to deterioration of the membrane; heat resistance at 80° C. and above which is the cell operating temperature range; and constant and high water retention of the membrane which enables it to keep low electrical resistance. In addition, the membrane which also plays the part of diaphragm is required to have outstanding mechanical strength and dimensional stability, as well as having no excessive permeability to the fuel hydrogen gas or methanol or oxygen gas.
When a reformed gas is to be used as fuel gas in fuel cells, it is desired that the fuel cell membrane be operated at 120° C. and higher temperatures in order to prevent poisoning of the platinum catalyst by carbon monoxide and other gases that are contained in very small amounts in the reformed gas. Currently used fluoropolymer-based fuel cell membranes have the disadvantage that the membrane dries up during cell operation, whereupon its water content decreases. If the membrane dries up, its internal resistance increases to lower its electrical conductivity, so the water content in the membrane must be controlled by various methods such as humidifying the fuel gas or operating the fuel cell at low enough temperatures of 60-80° C. [see J. J. Summer et al., J. Electrochem. Soc., 145, p. 107 (1998)]. It has therefore been pointed out that the conventional fluoropolymer-based fuel cell membrane requires a complicated fuel cell system. On the other hand, a membrane whose water content will not decrease if it is operated at 120° C. and higher is said to have remarkable advantages including less catalyst poisoning, elimination of the system for humidifying the fuel gas, an increased rate of electrode reactions, and a reduced size of the heat exchange system.
Early models of the polymer ion-exchange membrane fuel cell used a hydrocarbon-based polymer ion-exchange membrane produced by copolymerization of styrene and divinylbenzene. However, this ion-exchange membrane, being very poor in durability due to low oxidation resistance, was not highly feasible and was later replaced extensively by perfluorosulfonic acid-based membranes such as DuPont's Nafion®.
The conventional fluorine-containing polymer ion-exchange membranes such as Nafion® have outstanding chemical durability and stability but, on the other hand, it has been pointed out that their ion-exchange capacity is as small as about 1 meq/g and, due to insufficient water retention (low percent water content) at elevated temperature, the ion-exchange membrane will dry up, thereby impedes proton conduction or, in the case of using methanol for fuel, causes swelling of the membrane or cross-over of the methanol. If, in order to increase the ion-exchange capacity, one attempts to introduce more sulfonic acid groups, the membrane, due for example to its own characteristics and the absence of a cross-linked structure in polymer chains, will swell and its strength drops markedly. In addition, methanol used for fuel will cause swelling of the membrane.
Therefore, with the conventional fluorine-containing polymer ion-exchange membranes, the quantity of sulfonic acid groups had to be adjusted to small enough levels to guarantee the required membrane strength, so that one could only produce membranes having ion-exchange capacities of no more than about 1 meq/g. As another problem, fluoropolymers have only low affinity between the fluorine and water molecules and the chance of drying up increases with the cell operating temperature; this has made it extremely important to control the water content in the membrane by various methods such as humidifying the fuel gas or adjusting the operating temperature to between 60 and 80° C.
In addition, the fluorine-containing polymer ion-exchange membranes such as Nafion® have the problem of difficult and complex monomer synthesis; what is more, the process of polymerizing the synthesized monomers to produce the intended polymer membrane is also complex and yields a very expensive product, thereby presents a large obstacle to realizing a commercial proton-exchange membrane fuel cell that can be installed on automobiles and other equipment. Hence, efforts have been made to develop low-cost, yet high-performance electrolyte membranes that can be substituted for Nafion® and other conventional fluorine-containing polymer ion-exchange membranes.
In the field of radiation-induced graft polymerization which is closely related to the present invention, attempts are being made to prepare solid polymer electrolyte membranes by grafting onto polymer membranes those monomers into which sulfonic acid groups can be introduced. The present inventors conducted intensive studies with a view to developing such new solid polymer electrolyte membranes and found that solid polymer electrolyte membranes characterized by a wide range of controllability of ion-exchange capacity could be produced by first introducing a styrene monomer into a poly(tetrafluoroethylene) film having a cross-linked structure by radiation-induced graft polymerization reaction and then introducing sulfonic acid groups into the graft chains. The present inventors filed a patent application for the solid polymer electrolyte membrane having such characteristics and a process for producing it (JP 2001-348439 A). However, this ion-exchange membrane was composed of styrene units, so when it was supplied with an electric current for a prolonged period of time at 80° C. which was rather high temperature for the operation of fuel cells, the graft chain portion was partly oxidized to cause gradual decrease in the ion-exchange capacity of the membrane.
Membranes based on fluoropolymers other than crosslinked poly(tetrafluoroethylene) have been proposed, and an example is an ion-exchange membrane produced by first introducing a styrene monomer into an ethylene-tetrafluoroethylene copolymer through a radiation-induced graft polymerization reaction and then introducing sulfonic acid groups; the thus prepared membrane functions as an ion-exchange membrane for use in fuel cells (see JP 9-102322 A). However, this graft ethylene-tetrafluoroethylene copolymer membrane has the same drawback as described above and if a large electric current is applied to it for a prolonged period, the polystyrene graft chain portion undergoes oxidative deterioration and the ion-exchange capacity of the membrane will decrease gradually.
The present invention was accomplished in order to solve the aforementioned problems of the prior art. Fluoropolymer-based ion-exchange membranes which are currently in the mainstream have the disadvantage of being low in electrical conductivity and having low percent water content; the conventional graft polymerized polymer ion-exchange membranes have the disadvantage of low oxidation resistance.